It is a known fact that an image is a collection of pixels and that each pixel has a certain grey level encoded over a certain number of bits. If, for example, encoding of grey levels is effected over 8 bits, the grey levels can assumed any value between 0 and 255. It is assumed (hereinafter) that the value 255 corresponds to a bright pixel and the value 0 to a black pixel.
It will be noted that the detection of motion in a sequence of images implies that there is a difference between two consecutive images, such that the movable portions and the stationary portions become apparent. In a broad and general manner, it is known from the prior art, from the publication by CASASENT, Subpixel Target Detection and Tracking, SPIE, Vol. 726, 1986, to deal with detection in image processing by splitting the procedure into two steps:
(a) an extraction step; and PA1 (b) a tracking or validation step involving motion filtering. PA1 (a) deriving differential images D ( . . . , D.sub.n-1, D.sub.n,D.sub.n+1, . . . ) from succeeding images R; PA1 (b) spatially low pass filtering the differential images D to obtain a sequence of filtered differential images F( . . . , F.sub.n-1, F.sub.n, F.sub.n+1 . . . ), PA1 (c) for each pixel from a plurality of pixels (i,j) and for each of a plurality of directions (d.sub.x,d.sub.y), forming a sum of intensities from a number of successive images of the sequence of filtered differential images F.sub.n-m, where m ranging from 0 to M, at pixel locations (i+md.sub.x, j+md.sub.y) located, as the image is further back in time, along the direction progressively outwards from the pixel; and PA1 (d) for each pixel of the plurality of pixels, determining an intensity representative for the maximum size among the sums among the plurality of directions, the intensities for each pixel collectively forming an image G.sub.n.
From the prior art, a filtering method is known, for example, which includes the step of thresholding the difference between two images, in which consequently one single differential image is used, followed by a path tracking procedure. It appears that after a simple extraction, one is induced to take a binary decision.
In addition, from the publication of the prior art, by N. C. MOHANTY, Computer Tracking of Moving Point Targets in Space, Trans. PAMI, Vol. 3, no. 5, 606-611, 1981, three-dimensional linear filtering methods are known using large supports. Filters of that type have the property of allowing the total preservation of the data as it regards the grey levels. However, the volume of preserved data is very large, and the calculations required by a method including this type of filtration are absolutely enormous. This type of method is consequently only applicable to very small images.
The working context of the above-mentioned methods involve a number of problems. In the first place, the images are beset with noise, i.e., the differential image contains contributions due to noise which might give the impression that there are moving objects or targets.
In the second place, the images must be registered. This is achieved automatically when the camera recording the images to be processed is immobile. When the camera is mobile, for example, it is mounted on a moving vehicle, or it is mounted on a mast which Sways in the wind, the recorded images must be registered relative to the other images in such a manner that the background, for example, appears stationary in the sequence of images. This re-adjustment is effected in a manner known per set, which consequently does not form part of the invention. But it has been found that the re-adjustment procedure is not always perfect. It generally leads to registration errors of the order of one pixel, which means that between consecutive images, systematic errors of the order of one pixel appear in the coincidence of corresponding portions of the images. This results in a differential image in which the contribution of these registration errors is noticeable, and thus may be taken for moving objects. It is, on the other hand, also possible that in the registration procedure significant occasional errors appear which extend over a non-negligible portion of consecutive images in the image sequence. The contribution of these occasional errors, due to the registration procedure, also form an error source in the detection of slow moving targets.
In the third place, among the targets which may appear in the images, there are a certain number of targets which one does not want to detect, for example, swaying trees or moving clouds, or background portions subjected to variations in the illumination. In accordance with that fact, but contrary to those targets, one of the objects of the invention is the detection of moving objects having relatively continuous and relatively regular paths.